Lecture 15
The Uranium, Plutonium and Americium pE/pH Diagrams
Reading Assignment:
Homework:
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pE/pH diagrams are used to predict transport and mobility of metals in ground water. They are particularly important in understanding pollutant transport at contaminated waste sites, and in resolving the issues associated with burial of radioactive waste. The pE/pH diagrams of uranium, americium and plutonium will be used to illustrate these points.
We mentioned previously that, as a general rule, charged complexes are soluble in water and neutral complexes are not soluble. This is a key point to understanding pollutant transport or migration in ground water. Metals that form a charged species in ground water will dissolve and move in the direction of ground water flow. Metals that form a neutral species under the same conditions will not dissolve, and will therefore be immobile. This is why uranium behaves differently in the environment than plutonium.
Four pE/pH diagrams are shown for uranium in Figures 1-4. The simplest diagram, the U-O-H system, contains only four areas. The only important mobile form of uranium is UO22+, which would only exist in well aerated environments such as streams. U(OH)5- was not considered an important mobile form because it does not exist under any conditions found in the environment. Adding carbon to the system changes the pE/pH diagram significantly, as shown in Figure 2. Uranium forms three carbonate complexes, and two of these are soluble and occupy an area of the diagram formerly occupied by the insoluble U3O8. Clearly, the presence of carbonate in ground water would greatly affect the transport properties of uranium! Figure 3 shows the pE/pH diagram for the U-C-O-H system with an Fe-S overlay. This diagram is used by geochemists to show potential interactions that lead to pitchblende formation, and explain the occurrence of uranium ore deposits. The final uranium pE/pH diagram is the U-Si-C-O-H system, shown in Figure 4. Addition of silicon results in further significant changes. The UO2 and U3O8 sections of the diagram in Figure 2 are completely gone, and replaced with USiO4 and U(OH)5-. The significance point is that U(OH)5- is soluble and would exist at conditions found in ground water. The local soil conditions (i.e., silicon and carbonate abundances) would therefore have an important bearing on the ability of ground water to transport uranium metal.
Figures 5 and 6 show pE/pH diagrams for plutonium (Pu-C-O-H) in two different hydrologic environments. The usual situation for plutonium is shown in Figure 5, where only three plutonium forms are represented. This drawing is dominated by PuO2, which is insoluble and explains why under most conditions plutonium is immobile in ground water. Figure 6 show the pE/pH diagram for the same system in a situation with a high water to rock ratio. Under such conditions, plutonium would be mobile as Pu(OH)5-, but it has been observed that the mobile species is transformed to immobile PuO2 quickly in most natural settings.
Figures 7 and 8 show the pE/pH diagrams for americium (Am-O-H) and (Am-C-O-H). In the absence of carbonate, Figure 7, americium exists at Am3+ under most natural conditions. This species, of course, would dissolve and be mobile. Adding carbonate to the system, Figure 8, results in the formation of Am2(CO3)3 above a pH of 6. Am2(CO3)3 is insoluble and would be immobile.
Figure 15.1. pE/pH Diagram for U-O-H System
Figure 15.2. pE/pH Diagram for U-C-O-H System
Figure 15.3. pE/pH Diagram for U-C-O-H System with Fe-S Overlay
Figure 15.4. pE/pH Diagram for U-Si-C-O-H, 25 C, 1 Atm
Figure 15.5. pE/pH Diagram for Pu-(C)-O-H, 25 C, 1 Atm
Figure 15.6. pE/pH Diagram of the Amercium-O-H System
Figure 15.7. pE/pH Diagram for Am-C-O-H, 25 C, 1 Atm
Supplemental readings for this lecture:
1. Brookins, Douglas G., "Eh-pH Diagrams for Geochemistry," Springer-Verlag, New York, 1988.
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